The present invention relates to a magneto-optic recording system, in particular, relates to an overwritable type of system.
A prior optical disk has the advantage that the recording capacity is extremely large. However, it has the disadvantage that the recorded portion cannot be erased.
An erasable optical disk is described in page 1624-1628, IEEE Transactions on magnetics, vol. mag-21No. 5, September 1985. In that publication, an optical disk which is magnetized initially is illuminated by a laser beam under a bias magnetic field less than the anisotropic field in the opposite direction to the initial magnetic field in order to raise the temperature higher than the Curie temperature. When the optical disk is cooled, the magnetization of each cell is changed to the direction of the bias field. The information data relates to on/off of the laser beam so that the direction of the magnetization relates to the binary information data.
However, the disk has the disadvantage that the disk must be initialized in each recording operation by illuminating the disk with the laser beam in the initial field so that the disk is heated and is erased. In other words, the magnetization of the disk must be directed in one direction before recording, or the disk must be erased before recording. Therefore, no overwriting is possible in that prior disk, and so, high speed access to a disk was impossible. Therefore, an overwritable optical disk has been desired.
An overwritable optical disk is described in Proc. Int. Symp. on Optical Memory, 1987, Japanese Journal of Applied Physics, Vol. 26 (1987) Supplement 26-4, ppl149-154. In that publication, a recording layer is TbFeCo layer, and a recording head has both an optical head and a magnetic head. The recording layer is first heated by the optical head, and then, in the cooling duration of the layer, the layer is recorded magnetically.
However, the overwritable optical disk has the disadvantages that a recording pit is not symmetrical and so noise is caused, and that the recording magnetic field disturbs the positioning operation of the optical head.
Another overwritable optical disk is described in DE OS 3619618, in which a pair of magnetic layers with different Curie temperatures are used. A first layer is magnetically initialized in one direction, and new data is recorded by a laser beam which is modulated by a binary data under a bias field opposite to the initialization field. When data "1" is written, the laser beam is strong enough for heating two magnetic layers higher than the Curie temperatures, and so, the resultant magnetic field after cooling is defined by the bias magnetic field. On the other hand, when data "0" is written, the laser beam is controlled so that the first layer does not lose its magnetization, but the other magnetic layer loses its magnetization, and so, the resultant magnetic field after cooling is defined by the initial field.
However, the prior art has the disadvantage that the control of the operation is complicated, since the strength of a laser beam must be modulated by binary recording data.